This invention relates to direct-current motor drive circuits; more particularly, the invention relates to a safety drive circuit for a DC motor utilized in a liquid pumping system. The DC drive motor is mechanically linked to a reciprocable pump, wherein the pump must reciprocate at a variable rate in order to deliver liquids under predetermined pressure and volume conditions.
The invention is related to U.S. Pat. No. 4,397,610, owned by the assignee of the present invention, which is incorporated by reference herein. The prior patent discloses a pumping system utilizing a DC motor mechanically linked to a reciprocable pump, with a particular motor speed control circuit. The present invention is an improvement over the prior patent, particularly with regard to a number of different safety features which are incorporated into the invention.
Among the problems associated with the use of DC drive motors to drive reciprocable pumps are the problems of monitoring and controlling the electrical loading caused by the motor, since the motor is operated under variable speed conditions, ranging from stall conditions to relatively high rotational rates. Motor speed is regulated by a speed control circuit which monitors output liquid pressure and receives a fixed voltage set point input representative of the desired pressure operational point. The speed control circuit generates drive voltages to control motor speed and torque so as to equalize the output pressure at the preset operating point. The liquid delivery system to which the pump is connected may operate under flow conditions ranging from a blocked flow condition to a predetermined maximum delivery volume, all at a preset pressure requirement. Motor speed is controlled to deliver the liquid under any of these flow conditions, while maintaining the pressure at the desired setting. The motor speed control circuit is adversely affected by over-voltage conditions which may occur as a result of line voltage fluctuations, over current conditions which may result from a variety of factors, over-pressure conditions which may result from component malfunctions, motor stall conditions which may result under certain heavy loading conditions, and motor runaway conditions which may result from problems with the liquid delivery system. All of these conditions can lead to damage or destruction to the DC motor, or to the speed control circuits which drive the motor, and therefore it is important that safety features be incorporated to monitor for these conditions and to provide safe operating shutdown in the event any of these conditions occur.